\documentclass{article}

\usepackage{pgfplots}

\pgfplotsset{compat=1.6}

\begin{document}

\begin{tikzpicture}
	\begin{axis}[
		title=unless limits declared (x declared),
		xmin=0,xmax=1,
%		ymin=-3,ymax=3,
		axis equal,
		unit rescale keep size=unless limits declared,
	]
	\addplot coordinates {(0,-3) (1,3)};
		
	\end{axis}
\end{tikzpicture}
\end{document}

